In recent years considerable interest has been generated throughout the world in formable, high strength dual-phase sheet steel. The present invention is directed to such a type of steel. "Dual-Phase" sheet steels, introduced in the 1970's, offer the unusual combination of good formability and weldability in a steel of high tensile strength. The good formability is due to a low YS/TS ratio, a high work hardening rate which exists through to very high strains, and the high fracture strain. This combination of strength with formability and weldability is important, particularly to the automotive industry. Such combination offers the opportunity to the autobody fabricator to produce lighter parts while maintaining sufficient strength in such parts.
Microstructurally, dual-phase steels consist of a fine-grained equiaxed ferrite containing a coarse dispersion of a hard phase such as martensite or bainite. The steels can be produced by a number of different processes: (1) as-rolled, (2) batch-annealed, and (3) continuous-annealed in which the steel is annealed for a short time in the intercritical temperature range, rollowed by rapid cooling to transform the austenite to martensite/upper bainite.
Because of the strong interest in dual-phase steels, due in large measure to their very desirable properties, there is considerable published information on dual-phase steels. U.S. Pat. Nos. 4,222,796 (Davies), 4,129,461 (Rashid) and 4,033,789 (Hamburg et al) are exemplary of such publications and indicate the direction the art has taken on dual-phase steels.
The Davies patent discloses a method of making a dual-phase steel by adding 0.1-0.2% Mo to 0.3-0.5% Cr and/or 0.07-0.2% V, by weight, to a Mn--C steel, heating such steel to an intercritical annealing range of 740.degree.-830.degree. C. (1364.degree.-1526.degree. F.) for several minutes, and cooling at a rate less then 100.degree. C./sec (180.degree. F./sec).
Rashid, in his patent, presents some of the earliest work on the development of dual-phase steels. The patent is directed to a process for improving the formability of commercial HSLA steels, such as SAE 980X, which are essentially microalloyed low-carbon steels containing various additions from the group of carbide, nitride, and carbonitride formers, such as Ti, V, Cb. In the process, a steel of this type is heated to at least the lowermost eutectoid temperature of the steel for a time sufficient to at least partially transform the microstructure of the steel to austenite and to dissolve a substantial proportion of the microalloying additions into the austenite without appreciable grain growth and then cooling said steel to substantially lower the yield strength and improve the formability of the steel while maintaining its tensile strength. As a final step, the thus cooled steel is plastically deformed by an amount equivalent to at least 2% strain on the tensile stress-strain diagram for such a steel, to substantially increase the yield strength after the plastic deformation.
The Hamburg et al patent is directed to a method of heat treating a steel to improve its strength and formability. The process includes the steps of austenitizing a steel consisting essentially of from 0.04 to 0.17% carbon, 0.8 to 2.0% manganese, up to 1.0% silicon, up to 0.12% vanadium, up to 0.1% columbium, up to an effective amount of titanium to form titanium carbonitrides, 0.001 to 0.025% nitrogen, balance iron to a temperature above the Ac.sub.3 temperature of the steel. Thereafter, the steel is cooled at a rate of no more than about 70.degree. F./sec to about 850.degree. F., then further cooled at a rate of more than about 10.degree. F./sec. Such steel is characterized by a minimum ultimate tensile strength of 80,000 psi, a minimum tensile uniform elongation of 16%, and a microstructure of from about 10% to 35% MLB (martensite/lower bainite), balance essentially proeutectoid ferrite.
The disclosure of the present invention represents a further addition to the family of dual-phase steels that through the balancing of the steel's chemistry permits better or tighter control of the strength/ducility relationship. This balancing and the effects thereof will become more apparent from the specifications which follow.